No she didn't kill herself that ending was part of house's ending slide (and it had nothing to do with her personal quest).
Cassidy has two ending slides, one based off how you complete her personal quest and one based on the winning faction.
To clarify: This isn't a reference to her killing herself. The last words are referring to how she perceives New Vegas itself - she thinks House is holding too tightly to the Old World, so a "victory" at Hoover Dam to her is actually a step back into history and the old ways ("facing backwards the whole time"), not really moving into a new future. No confusion intended.
This came up before and Chris Avellone was kind enough to clairify.
This is Cassidy's ending slide for the peaceful option.
In the years following the destruction of Cassidy Caravans, NCR used evidence of the plot to blackmail the Crimson Caravan and the Van Graffs. NCR enacted strict trade laws with little resistance, strengthening their supply lines and their position in the Mojave.